Hi, correct me if I'm wrong, but quantum mechanics and special relativity seem to fit pretty well together. With Klein-Gordon's equation and Dirac's equation anyway, they seem to be able to live with each other quite happily. Are there cases in which they can't? Is this what Q Field theory deals with? What are the main problems or obstacles in this area, if any? I'm talking about Special R. only here, NOT General Relativity.